Answer:
(a) if the string is rigidly attached to the post it means that the reflected wave is upside down from the original. Thus they add to zero when they cross.
(b) if the string is freely attached to the post it means the reflected wave is rightside up. Thus when they cross the amplitudes add so 0.10 + 0.10 = 0.2 m.
Step-by-step explanation: